Acoustic monitoring systems and other stimulating and sensing electronic devices may be located on substrates, such as aircraft and other vehicles, bridges, buildings, machinery, pipelines, and other structures, to check for cracks, de-lamination, corrosion, or other degradation or damage. Many papers have been published showing these devices.
Lamb waves have long been used for nondestructive test, and a review article, “Review of IAR NDI Research in support of Ageing Aircraft,” incorporated herein by reference, by Fahr, Komorowski, Forsyth, Chapman, NDT.net—January 1999, Vol. 4 No. 1, and available at http://www.ndt/article/padndt98/12/12.htm, cites many references. The article provides a review of nondestructive test, including several ways of using the acoustic waves generated with piezoelectric transducers to test materials. The paper also sites research papers describing fatigue mechanisms of aging aircraft that can be tested with lamb waves to find such defects as corrosion, delamination, and cracks. The paper describes several ways of coupling piezoelectric transducers to the material under test but includes no description of the source of stimulation or data analysis. An extensive wiring harness between the piezoelectric transducers appears to be needed to connect the piezoelectric transducers to racks of electronics that provide the stimulating waveform and that receive and analyze the data.
“Structural Health Monitoring in Composite Materials Using Frequency Response Methods,” Kessler, et al, Nondestructive Evaluation of Materials and Composites, Proceedings of SPIE Vol. 4336 (2001), provides a “survey of candidate methods for the detection of damage in composite materials.”
“Design of piezoelectric-based structural health monitoring system for damage detection in composite materials,” Kessler and Spearing, Smart Structures and Integrated Systems, Proceedings of SPIE Vol. 4701 (2002), describes the use of Lamb wave techniques to test specimens containing damage and the use of piezoelectric patches as actuators and sensors.
“Optimization of Lamb Wave Actuating and Sensing Materials for Health Monitoring of Composite Structures, Kessler and Dunn, Smart Structures and Materials 2003: Smart Structures and Integrated Systems, Proceedings of SPIE Vol. 5056 (2003), further describes the effectiveness of Lamb wave methods for health monitoring of composite structures and provides results concerning the sensing patch and algorithms for filtering resulting signals.
“Packaging of Structural Health Monitoring Components,” Kessler et al., Smart Structures and Materials 2004: Smart Structures and Integrated Systems, Proceedings of SPIE Vol. 5391 (2004) further describes “the ability of Lamb waves methods to provide reliable information regarding the presence, location and type of damage in composite specimens.” The paper also provides ways to package a wired structural health monitoring device with components for mounting on a substrate to be tested. The package includes leads extending to a function generator and to a data acquisition device. The paper mentions the need for further research to produce a fully functional SHM system that would “include the batteries, wireless communication and local storage device as well as system software.”
Such systems have required separate pairs of wires 20, 20′ extending from each acoustic actuator/sensor 22 of array 23 of such actuator/sensors to its own rack 24 of large electronic devices, as shown in FIGS. 1a, 1b, and 2. Each rack 24 includes programmable function generator 26 and digital storage oscilloscope 28. At least one computer 30 is also needed, and that computer 30 can serve all the racks 24 of electronic devices and all the actuator/sensors 22. Each programmable function generator 26 provides a desired waveform to the actuator/sensor 22 acting as an actuator 27. Digital storage oscilloscope 28 provides memory for receiving and recording data sensed by those actuator/sensors 22 acting as sensors 29. Computer 30 then receives the data from digital storage oscilloscopes 28 and provides data analysis, as shown in FIG. 2.
One system, available commercially from Acellent, Inc., Sunnyvale, Calif., can be used for analyzing substrate 32 on which it is mounted. The system includes array 23 of actuator/sensors 22 and wiring integrated on insulating film 40 extending to connector 42 to which a wiring harness, including wires 20, 20′ for each actuator/sensor, were connected. Wires 20, 20′ within this wiring harness could extend to the racks 24 of electronics located off of the substrate, such as schematically illustrated in FIG. 2. Alternatively, the racks of electronics were shrunk to a “smart suitcase,” as shown in product literature provided by Acellent available on line at http:/www.acellent.com/products_layer.htm in which 30 sensor/actuators were wired to the smart suitcase. Thus, a wiring harness with 60 wires would be connected from an array of Acellent system actuator/sensors 22 to the electronics located in the smart suitcase. That electronics, including both the signal generation and the data acquisition hardware, was provided in the suitcase, which weighed about 18 pounds.
The need for separate pairs of wires 20, 20′ for each actuator sensor 22 and the need for a separate rack 24 of large electronic devices for each actuator/sensor 22 or for a smart suitcase with those electronic devices has limited the number of actuator/sensors 22 that could be provided on a structure, the types of structures that could be monitored, the frequency of monitoring, and the duration of monitoring. The ability to monitor during actual operation of certain structures, such as aircraft wings was also limited.
A source of electrical stimulation has been applied to a material through electrodes or through a device, such as capacitor, that includes a portion of the material. Impedance measurements have been used to provide important information about the material. For example, U.S. Pat. No. 6,911,828 to Brossia et al., provides a system for monitoring the effectiveness of a coating on a substrate surface and indicating the failure of the coating to adequately protect the surface from corrosion, degradation, and the like. The system includes a sensor array positioned in contact with the coating utilizing a number of sensor electrodes connected to a single integrated circuit or a number of separate individual sensor circuits. The electrodes of the sensor array make measurements of the electrochemical impedance characteristics of the coating and provide such data by way of telemetry to a data interrogation device that is periodically be placed in proximity to the sensor array. The interrogation device may serve to both power the sensor array and trigger it to acquire data. A nominal parameter N′, which is the product of the impedance magnitude and the phase angle, is utilized as a direct indication of the resistance and capacitance characteristics of the coating and therefore a direct indication of the coatings effectiveness. Variations in the frequency of the interrogation signals transmitted from the data interrogation device signal frequency would permit not only the discreet interrogation of a single sensor at a time but also the acquisition of signal data associated with a variety of sensor frequencies.
A paper, “Overview of Piezoelectric Impedance-Based Health Monitoring and Path Forward,” by Gyuhae Park, et al., The Shock and Vibration Digest, Vol. 35, No. 6, November 2003, p. 451-463 (“the Park paper”), incorporated herein by reference, describes an approach to piezoelectric impedance-based health monitoring with piezoceramic (PZT) materials.
The impedance based health monitoring system is based on the recognition that “the electrical impedance of the PZT is directly related to the mechanical impedance of the host structure, allowing the monitoring of the host structure's mechanical properties using the measured electrical impedance. Consequently, any changes in the electrical impedance signature can be considered an indication of changes in the structural integrity.” In this approach changes in properties of the host structure can be detected simply by monitoring the electrical impedance of the PZT patch over a range of frequencies of the stimulating vibratory signal.
The Park paper describes using “piezoelectric sensors/actuators to acquire dynamic responses of a structure” over the range of frequencies and discusses the relationship between defect size and frequency of signal. The Park paper mentions that the sensors can be used to check the perfection of the bonding of the piezoelectric sensors/actuators to the structure.
The Park paper lists alternate non destructive evaluation techniques, including ultrasonic technology, acoustic emission, magnetic field analysis, penetrant testing, eddy current techniques, X-ray analysis, impact-echo testing, global structural response analysis, and visual inspections. It notes that pulse-echo techniques can be nicely integrated with the impedance method. The Park paper also mentions deploying a network of sensors, in which “each individual PZT patch is activated as an actuator in turn, and the rest of the PZTs act as sensors scanning a large area.”
Under a discussion of “future issues,” the Park paper states, “the development of standalone, miniaturized impedance measurements system should be pursued.” It also notes that the deployment of a dense array of sensors “potentially produces difficulties and complexities in data acquisition and processing. The efficient management of data from a largely distributed sensing system is an important and challenging issue.”
The Park paper also notes a “drawback in using multiple sensors is the wiring harnesses needed to connect the sensors to signal processing and computers for obtaining the required information regarding the health of the host systems. Therefore, the integration of wireless telemetry systems into the impedance-measuring unit is imperative to manage and operate the sensing devices.”
The Park paper also notes, “recognizing the fact that it takes much more energy to transmit data than to perform the local computation, it is important to embed local processing capabilities at the sensors and use a telemetry system to send only essential data.”
The Park paper also states, “another important issue in designing such a system is the management of the power consumption.” It recognizes a proposal “to use the voltages generated by natural or ambient vibration in a system. The proposed device is coupled with the PZT, stores the electric energy in a capacitor (or recharges a battery), regenerates it through discharge in a controlled signal for diagnostics and runs the telemetry of the wireless sensing system.”
The Park paper states that “a device, which incorporates algorithms in the areas of impedance acquisitions, embedded signal processing, telemetry, and power management mentioned above into one package, will provide significant potential in structural health monitoring and damage prognosis efforts. This device would have a network of sensors in control and can act as a station between sensor networks and the central health monitoring station.”
However, such a desired device has not been available for monitoring structures. Nor has a way to make such a device been provided. This description and such a device are provided in the present patent application. In addition, a better scheme is needed to reduce or eliminate the wiring and to reduce or eliminate the racks of large electronic devices, and this scheme is also provided by this patent application.